Method of recycling a catalyst in a reaction involving the direct oxidation of cyclohexane into adipic acid

ABSTRACT

PCT No. PCT/FR95/00944 Sec. 371 Date Feb. 26, 1997 Sec. 102(e) Date Feb. 26, 1997 PCT Filed Jul. 13, 1995 PCT Pub. No. WO96/03365 PCT Pub. Date Feb. 8, 1996A process for recycling a catalyst containing cobalt including treating a reaction mixture obtained during the direct oxidation of cyclohexane to adipic acid by extracting at least some of the glutaric acid and succinic acid which are formed in the reaction.

This application was filed as a 35 U.S.C. §371 of PCT/FR95/00944, whichdesignated the United States.

The present invention relates to the field of the one-step oxidation ofcyclohexane to adipic acid, using a gas containing oxygen, in liquidphase and in he presence of a catalyst containing cobalt.

The direct oxidation of cyclohexane to adipic acid is a process whichhas been worked at for a long time, in particular on account of theobvious advantages there would be in converting cyclohexane into adipicacid, in a single step and without using an oxidant such as nitric acid,this compound generating nitrogen oxides which must then be treated inorder to avoid any contamination.

Thus, American patent U.S. Pat. No. 2,223,493, published in December1940, describes the oxidation of cyclic hydrocarbons to correspondingdiacids, in a liquid phase generally containing acetic acid, at atemperature of at least 60° C., using a gas containing oxygen and in thepresence of an oxidation catalyst such as a cobalt compound. This patentmakes provision for a separation of the adipic acid formed bycrystallization, but teaches nothing about the manner of recycling thecatalyst, in a new oxidation operation, and less still about theactivity which a catalyst that has been recycled one or more times wouldhave.

Patent Application WO-A-94/07833 describes a similar process, specifyingthat the solvent represents less than 1.5 mol per mole of cyclichydrocarbon, that the said solvent comprises an organic acid having onlyprimary or secondary hydrogen atoms and that the reaction is carried outin the presence of at least 0.002 mol of cobalt-based catalyst per 1000g of reaction mixture. At the end of the reaction, the diacid formed inisolated.

Patent Application WO-A-94/07834, filed on the same day as the abovepatent application, also describes the same process, but develops thephases for the treatment of the final reaction mixture. This treatmentconsists in separating the diacid formed, by cooling the mixture inorder to bring about precipitation of the said diacid, and in separatingby filtration the diacid from two liquid phases, a non-polar one whichis recycled, and a polar one which is also recycled after an optionalhydrolysis and a separation of an additional mount of diacid.

These various patents present solutions which allow the one-stepoxidation of cyclohexane to adipic acid with an industrially acceptableselectivity, but they do not address the specific problem of theprogressive and relatively rapid deactivation of the catalyst during itsrecycling.

Indeed, when the reaction mixture obtained from the cyclohexaneoxidation reaction is cooled in order to crystallize some of the adipicacid, and then filtered in order to separate out this precipitatedadipic acid, the filtrate thus obtained contains the catalyst, residualadipic acid, the reaction by-products (especially glutaric acid,succinic acid, cyclohexanol, cyclobextaone, hydroxycaproic acid andcyclohexyl esters), the arencted cyclohexane, the acetic acid solventand the water formed.

Some of these various compounds definitely influence the deactivation ofthe catalyst. In the process described in Patent ApplicationWO-A-94/07834, the larger part of these various compounds is recycled tothe oxidation, optionally after addition of further amounts ofcyclohexane and optionally after separation of an additional amount ofadipic acid remaining in the mixture. It turns out that when thecatalyst and the various by-products are recycled in a new oxidationreaction, relatively rapid deactivation of the catalyst is observed.Thus, it is indicated in the various examples of WO-A-94/07834 that therate of formation of the adipic acid decreases by 26% to 43% after fourcycles of the catalyst.

One of the subjects of the present invention is thus to allow therecycling of the cobalt catalyst with little or no deactivation.

In order to achieve this aim, the process of the invention includes astep for the treatment of the reaction mixture obtained during thedirect oxidation of the cyclohexaue to adipic acid, consisting of anextraction of at least some of the glutaric acid and succinic acid whichare formed in the reaction.

A first subject of the invention consists first of all of a process forrecycling a catalyst containing cobalt, in a reaction for the directoxidation of cyclohexane to adipic acid, in a solvent comprising atleast one aliphatic carboxylic acid having only primary or secondaryhydrogen atoms, by a gas containing oxygen, the said recycling processbeing characterized:

in that the reaction mixture derived from a prior operation for theoxidation of the cyclohexane to adipic acid in treated in order toremove the more volatile compounds, optionally after the crystallizationand separation of at least some of the adipic acid which it contains,

in that the residue obtained is extracted using a solvent, chosen fromketones, alcohols, esters, various mixtures thereof or mixtures ofhydrocarbon and carboxylic acid, this solvent being capable ofdissolving all or a large part of the diacids contained in the saidresidue,

in that the extraction residue thus obtained, containing the larger partof the cobalt catalyst, is used in a new operation for the oxidation ofcyclohexane to adipic acid, after addition of the necessary complementsof cyclohexane, of carboxylic acid and, where appropriate, of cobaltcatalyst.

In the present text, the term "large part" means at least 50% by weightof the total amount of the compound or compounds under consideration.

The reaction mixture may conveniently be treated by distillation, atatmospheric pressure or at reduced pressure, of the more volatilecompounds which are especially the unreacted cyclohexane, the carboxylicacid serving as solvent in the oxidation reaction, the water formed andcertain intermediate compounds such as cyclohexanol and cyclohexanone.An mentioned previously, this treatment may be preceded by thecrystallization of some or all of the adipic acid, via cooling of thereaction mixture, and by its separation, for example via filtration orcentrifugation.

The ketones which may be used to extract the catalyst from the residueobtained are especially acetone, methyl ethyl ketone and cyclohexanone,cyclohexanone being preferred since it can, if required, be reused inother stages of the process, for example it can be recycled in theoxidation.

The alcohols which may be used to extract the catalyst from the residueobtained are especially 1-propanol, 2-propanol, 1-butanol, 2-butanol,tert-butanol and cyclohexanol, cyclohexanol being preferred since itcan, if required, be reused in other stages of the process, for exampleit can be recycled in the oxidation.

The esters which may be used to extract the catalyst from the residueobtained are especially esters derived from the alcohols mentioned abovewith aliphatic carboxylic acids having only primary or secondaryhydrogen atoms, such as those which are used in the cyclohexaneoxidation reaction.

Mixtures of several of these extraction solvents may, of course, beused, especially mixtures of cyclohexanol and cyclohexanone (sometimesreferred to as olone). Other solvent mixtures may also be suitable, forexample mixtures of aliphatic or cycloaliphatic hydrocarbons and ofcarboxylic acids, which will be defined below.

A second subject of the invention also consists of a process forrecycling a catalyst containing cobalt, in a reaction for the directoxidation of cyclohexane to adipic acid, by a gas containing oxygen, thesaid recycling process being characterized:

in that the reaction mixture, obtained from a prior operation ofoxidation of the cyclohexane to adipic acid, from which mixture at leastsome of the intermediate oxidation products, especially such ascyclohexanol and cyclohexanone, some carboxylic acid solvent and waterhas been separated and from which mixture at least some of the adipicacid formed has been recovered by crystallization, is subjected to atleast one extraction using at least one cosolvent or using a mixturecomprising a coaolvent and a carboxylic acid,

in that there are separated, on the one hand, a mixture containing atleast some of the cobalt catalyst, some of the carboxylic acid andpossibly residual amounts of other compounds and, on the other hand, asolution containing the cosolvent and at least some of the glutaric acidand succinic acid which are formed in the oxidation reaction, as well assome carboxylic acid,

and in that the mixture containing at least some of the cobalt catalystis used in a new operation for the oxidation of cyclohexane to adipicacid, optionally after the addition of extra cobalt catalyst.

The cosolvents which may be used in the process according to the secondsubject of the invention are generally chosen from hydrocarbons,especially aliphatic and cycloaliphatic hydrocarbons, ketones andalcohols.

Among the hydrocarbons, there may be mentioned hexane, heptane, octane,nonane, decane, undecane, dodecane and cyclohexane.

Among the ketones, those which have been mentioned for the extraction ofthe residue in the process according to the first subject of theinvention may be used, cyclohexanone being preferred.

Among the alcohols, those which have been mentioned for the extractionof the residue in the process according to the first subject of theinvention may be used, cyclobexanol being preferred.

It is preferable to use cyclohexane as cosolvent in the processaccording to the second subject of the invention, since this facilitatesthe treatment of the various solutions obtained during the process andtheir possible recycling in the oxidation reaction.

The crude reaction mixture which, after certain separation operations,is used in the process of the invention, is obtained from the oxidationknown per se of cyclohexane, by a gas containing oxygen, in a liquidmedium comprising a carboxylic acid and in the presence of a catalystcontaining cobalt.

For the preparation of this crude reaction mixture, reference may bemade to the processes described in the prior art, especially in theabovementioned U.S. Pat. No. 2,223,493. Thus, the initialcyclohexane/carboxylic acid weight ratio may, for example, be between0.1/1 and 10/1 and preferably between 0.5/1 and 3/1. The cobalt catalystpreferably comprises a cobalt compound which is soluble in the reactionmedium, chosen for example from cobalt carboxylates, preferably such ascobalt acetate tetrahydrate, cobalt chloride, cobalt bromide and cobaltnitrate.

The amount of catalyst, expressed as a weight percentage of cobaltrelative to the reaction mixture, is generally between 0.01% and 5% andpreferably between 0.05% and 2%, without these values being critical. Itis, however, a question of having a sufficient activity while not usingexcessive amounts of a catalyst which must then be separated from thefinal reaction mixture and recycled.

Besides cobalt, the catalyst may also contain other compounds based onmetals such an manganese and/or copper and/or cerium and/or vanadium.

It is advantageous also to use an initiator compound for the oxidationreaction, for example such as a ketone or an aldehyde. Cyclohexanone,which is a reaction intermediate, is most particularly mentioned. Theinitiator generally represents from 0.01% to 20% by weight of the weightof the reaction mixture used, without these proportions having acritical value. The initiator is especially useful when starting theoxidation and when the cyclohexane is oxidized at a temperature below120° C. It may be introduced at the start of the reaction.

The carboxylic acid acting an solvent in the cyclohexane oxidationreaction is more particularly a saturated aliphatic carboxylic acidhaving from 2 to 9 carbon atoms and having only primary or secondaryhydrogen atoms.

Acetic acid is preferably used as solvent for the cyclohexane oxidationreaction. In the rest of the present description, acetic acid will bereferred to for convenience as the carboxylic acid used in the varioussteps of the process.

The oxidation may also be carried out in the presence of waterintroduced in the initial stage of the process.

The cyclobexane oxidation reaction is generally carried out at atemperature of from 60° C. to 180° C. and preferably of from 70° C. to120° C.

The pressure is not a critical parameter of the reaction and isgenerally between 10 kPa (0.1 bar) and 10,000 kPa (100 bar).

Before performing the extraction operation according to the invention,the crude reaction mixture obtained from oxidation of the cyclohexane insubjected to various operations for the separation of some of itsconstituents.

According to a first variant of the process according to the secondsubject of the invention, the crude reaction mixture may first besubjected to cooling to a temperature of 16° C. to 30° C. for example,which brings about the crystallization of at least some of the adipicacid formed. A three-phase medium is thus obtained comprising a solidphase essentially consisting of adipic acid, an upper cyclohexane liquidphase essentially containing the unreacted cyclohexane and a loweracetic liquid phase essentially containing the acetic acid, the waterformed, adipic acid, the cyclohexane oxidation intermediates such ascyclohexanol, cyclohexanone and hydroxycaproic acid, by-products such asglutaric acid and succinic acid, and the cobalt catalyst. The mediumobtained by cooling the reaction mixture may, where appropriate, be atwo-phase medium, that is to say that it contains only the precipitatedadipic acid and the acetic phase, if the conversion of the cyclohexaneduring the oxidation is complete or almost complete.

After filtration or centrifugation of the solid, the two liquid phasesconstituting the filtrate or the centrifugate are separated by settling,if need be: the cyclohexane phase, which contains small amounts ofcyclohexane oxidation products, may be recycled in a new oxidationreaction.

It may also be advantageous to concentrate the reaction mixture, priorto the adipic acid crystallization operation; a single acetic liquidphase may then be found, during the precipitation of the adipic acid.

According to a second variant of the process according to the secondsubject of the invention, the final crude reaction mixture may beremoved while hot, for example at a temperature which may be up to 75°C. The reaction mixture then separates into two liquid phases bysettling: an upper cyclohexane phase essentially containing theunreacted cyclohexane and a lower acetic liquid phase essentiallycontaining the acetic acid, the adipic acid, the water formed, thecyclohexane oxidation intermediates such as cyclohexanol, cyclohexanoneand hydroxycaproic acid, by-products such as glutaric acid and succinicacid, and the cobalt catalyst.

As in the first variant, the two liquid phases are separated bysettling: the cyclohexane phase, which contains small amounts ofcyclohexane oxidation products, may be recycled in a new oxidationreaction.

The comment made above for the first variant is also valid for the othervariants: if the cyclohexane used in the oxidation is virtually allconverted, there cannot be two liquid phases there, but only a singleacetic phase.

After possible eoncentration, the acetic phase is then cooled to atemperature of 16° C. to 30° C. for example, which causes thecrystallization of at least some of the adipic acid formed, which isthen separated by filtration or centrifugation. This adipic acid may bepurified by recrystallization from a suitable solvent, which mayadvantageously be acetic acid or water. When acetic acid is used as therecrystallization solvent, it may then be added to the acetic phaseobtained above.

According to a third variant of the process according to the secondsubject of the invention, a water/cyclohexane azeotropic mixture may bedistilled off during the oxidation reaction and, after separation ofthis mixture by settling, the cyclohexane may then, if so desired, bereintroduced into the reactor. This allows at least some of the water tobe removed from the reaction mixture. The final crude reaction mixturemay then be treated according to the first variant described above, thatis to say by cooling in order to precipitate the adipic acid, filtrationor centrifugation. The consequence of removing the water is to avoid thepossible separation of the liquid part into two distinct phases onsettling. The single liquid phase obtained is then treated in the sameway as the acetic phase.

The acetic phase, obtained in one or other of these three variants forthe treatment of the final crude reaction mixture, is subjected to anextraction with cyclobexane, either such as it is obtained afterseparation by settling (especially in the case where the water has beenremoved therefrom), or preferably after having been concentrated byheating to a temperature of 30° C. to 80° C. at reduced pressure. Thisconcentrating makes it possible to remove some of the acetic acid, atleast the majority of the water and at least some of the light compoundswhich may be present, such an some of the cyclohexane, of thecyclohexanol or of the cyclohexanone. The compounds thus separated fromthe acetic phase may be recycled in the cyclohexane oxidation step,either in total or after separation of at least some of the water whichthey contain. Generally, concentrating reduces the acetic phase to avolume representing from 80% to 10% of its initial volume, these valuesbeing given merely as examples. One variant consists in concentrating todryness the acetic phase, that is to say in removing all of the aceticacid which it contains.

Partial concentration of the acetic phase may also allow, if it isfollowed by a cooling under the conditions indicated above for thecrystallization of the adipic acid, an additional amount of adipic acidto be precipitated.

The concentrated or non-concentrated acetic phase is extracted eitherwith cyclohexane alone or with cyclohexane/acetic acid mixtures. Theessential point is, however, that the mixture consisting of the aceticphase subjected to the extraction and of the cyclohexane should have anoverall cyclohexane/acetic acid weight ratio between 1/1 and 50/1 andpreferably between 2/1 and 15/1. This means that, depending on thecomposition of the acetic phase obtained after the various treatments ofthe reaction mixture which are described above, the acetic acid requiredto extract the composition thus defined with a cyclohexane/acetic acidmixture will consist partly or totally of the acetic acid contained inthe acetic phase or will be introduced with the cyclohexane in the casewhere the acetic acid of the said acetic phase has been removed.

The extraction may be performed one or more times or, within the contextof a continuous process, by the usual industrial techniques. It may becarried out at a temperature ranging up to the boiling point of thesolvent or solvents used. The extraction is generally performed between10° C. and 80° C. and preferably between 50° C. and 80° C.

The extraction operation gives, on the one hand, a solution whichcontains at least some of the glutaric acid and succinic acid which itis desired to separate out, as well as residual amounts of otherby-products which may still remain, such as lactones, esters andproducts of over-oxidation, and, on the other hand, a mixtureessentially containing the cobalt catalyst. This mixture is generallyseparated by settling.

The cobalt catalyst thus separated out is recycled in a new cyclohexaneoxidation reaction, optionally after a further addition to compensatefor the losses suffered during the various treatments of the reactionmixture obtained from the cyclohexane oxidation.

This catalyst remains as active as the new catalyst used in the firstcyclohexane oxidation operation and it may thus be recycled a largenumber of times without considerably decreasing its activity and theselectivity of the reaction to form acetic acid.

A third subject of the invention consists of a continuous process forthe oxidation of cyclohexane to adipic acid, using a gas containingoxygen, in a liquid medium comprising a carboxylic acid as solvent andin the presence of a catalyst containing at least cobalt, characterizedin that it includes the following steps:

a) the actual oxidation of the cyclohexane to adipic acid,

b) the reaction mixture derived from the oxidation of the cyclohexane toadipic acid is treated in order to remove the more volatile compounds,optionally after crystallization and separation of at least some of theadipic acid which it contains,

c) the residue obtained is extracted using a solvent chosen fromketones, alcohols, esters, various mixtures thereof or mixtures ofhydrocarbon and carboxylic acid, which solvent is capable of dissolvingall or a large part of the diacids contained in the said residue,

d) the extraction residue thus obtained, containing the larger part ofthe cobalt catalyst, is used in a new cyclohexane oxidation operationa).

Each of the various steps of this continuous process has been outlinedabove and reference may be made to this description for the particularembodiments of the process.

A fourth subject of the invention consists of a continuous process forthe oxidation of cyclohexane to adipic acid, using a gas containingoxygen, in a liquid medium comprising acetic acid as solvent and in thepresence of a catalyst containing at least cobalt, characterized in thatit includes the following steps:

a) the actual oxidation of the cyclohexane to adipic acid,

b) either: b1) cooling of the crude final reaction mixture, optionallyafter concentrating, in order to crystallize at least some of the adipicacid, separation of the said adipic acid by filtration or centrifugationand then, if necessary, separation by settling of the filtrate or of thecentrifugate obtained into a cyclohexane phase and an acetic phase,

or: b2) removal of the crude final reaction mixture while hot, the maidreaction mixture being a two-phase or a monophase mixture, its possibleseparation by settling into a cyclohexane phase and an acetic phase,cooling of the acetic phase which has been separated by settling or,where appropriate, of the single acetic phase so as to crystallize atleast some of the adipic acid, and separation by filtration orcentrifugation of the said adipic acid and of the acetic phase,

or: b3) removal of the water from the reaction mixture during theoxidation step a), by distillation of the cyclohexane/water azeotropeand optional reintroduction of the distilled cyclohexane into thereactor, followed by treatment of the crude final reaction mixtureaccording to the variant b1), the said treatment giving, in this came, asingle liquid phase which is treated in the same way as an acetic phasefor the remainder of the process,

c) concentration of the acetic phase in order to remove therefrom atleast the majority of the water, if this has not been done in thevariant b3),

d) extraction of the acetic phase using cyclohexane or acyclohexane/acetic acid mixture in an amount such that, in thecyclohexane/acetic phase mixture, the overall cyclohexane/acetic acidweight ratio is between 1/1 and 50/1 and preferably between 2/1 and15/1,

e) separation of a cyclohexane solution containing at least some of theglutaric acid and succinic acid, on the one hand, and of a mixtureessentially containing the cobalt catalyst, on the other hand,

f) recycling of the mixture containing the cobalt catalyst in a newoxidation reaction a).

Each of the various steps of this continuous process has been outlinedabove and reference may be made to this description for the particularembodiments of the process.

The examples which follow illustrate the present invention.

EXAMPLES 1 TO 5

The compounds given below are charged, at room temperature, into atitanium-sleeved 1.5 liter autoclave which has been pre-purged withnitrogen and which is fitted with a six-blade double turbine and variousopenings for introduction of the reagents and the fluids or for removalof the reaction products and the fluids:

cobalt acetate tetrahydrate: 4.0 g (16 mmol)

acetic acid: 359 g (5.98 mol)

cyclohexane: 289.7 g (3.45 mol)

acetaldehyde: 1.2 g (27.3 mmol).

After closing the autoclave, the nitrogen pressure is brought to 20 bar,stirring is started at 800 revolutions/min and the temperature isbrought to 102° C. over 29 min. The nitrogen is then replaced by 20 barof depleted air (5.85% oxygen). The outlet gas flow rate is set at 250liters/hour.

After about 10 min of induction, during which there is no oxygenconsutption, the temperature rises suddenly to 106° C. and the oxygenstarts to be consumed. When the oxygen content of the air at theautoclave outlet reaches 1%, the supply of depleted air at an oxygencontent of 5.85% is replaced by a supply of air at an oxygen content of11.35%. The oxygen content at the reactor outlet remains below 1%throughout the test. The average temperature in the autoclave ismaintained at 106°-107° C.

When 50 liters of oxygen have been consumed, the outlet valve and theair supply are closed. At the same time, the temperature in theautoclave is gradually brought to 75° C.

The reaction mixture is then recovered using a withdrawal valveconnected to a dipping tube, and is then rapidly cooled to about 20° C.

A three-phase medium is obtained, consisting of crude adipic acid whichhas precipitated (49 g) , an acetic phase (451.4 g) and a cyclohexanephase (143.2 g) the autoclave is rinsed with acetic acid and thisrinsing liquid is added to the acetic phase. The contents of a trap (5.5g) placed after a condenser are added to the cyclohexane phase.

On recrystallization from acetic acid, 294.5 mal of recrystallizedadipic acid are obtained, as well as 38.5 mmol of adipic acid, 1.6 mmolof glutaric acid and 1.2 mmol of succinic acid which are dissolved.About 5% of the catalyst, which has been entrained by precipitation ofthe adipic acid, are recovered in the acetic acid from therecrystallization and this acetic solution is added to the acetic phase.

The acetic and cyclohexane phases are assayed by gas chromatography.

The results of Example 1 and of the comparative tests or examples whichfollow are expressed in the following way:

degree of conversion (DC) of the cyclohexane: mol % of the cyclohexaneconverted into the various compounds of the final reaction mixture;

selectivity (RT) towards product P: moles of product P×100/moles ofcyclohexane converted:

linearity of diacids formed (Lty): moles of adipic acid formed×100/sumof the moles of the adipic, glutaric and succinic acids formed.

The acetic phase contains virtually all of the acids and lactones formedand the very large majority of the cyclohexanol, the cyclohexanone andthe cyclohexyl acetate which are formed, as well as the water andvirtually all of the catalyst.

The acetic phase is concentrated by heating to 50° C., at reducedpressure (4 kPa) in order to remove the water and some of.the aceticacid, cyclohexane, cyclohexanol and cyclohexanone, until a solutioncontaining 110 g of acetic acid and 35 g of products (essentially thediacids and the cobalt catalyst) is obtained.

This concentrated acetic phase in treated with 1300 g of cyclohexane at70° C. A red precipitate and a colourless supernatant liquid appear, theliquid being removed while hot (60° C.).

The supernatant contains most of the oxidation products (diacids,cyclohexanol, cyclohexanone, esters and lactones) whereas theprecipitate contains more than 95% of the cobalt (amount determined byassay on a sample) which is returned to the reactor with the extracobalt, cyclohexane and acetic acid nocessary to reestablish theproportions indicated above for the first test.

The catalyst is successively recycled 4 times (Examples 2 to 5).

The results obtained with Examples 1 to 5 are collated in Table 1 below.

In this table, the following new abbreviations are used:

AdOH for adipic acid

olone for the cyclohexanol/cyclohexanone mixture

O2 for the maximum amount of oxygen consumed in liters/hour (this valuerepresents the maximum rate of oxidation)

Peff for AdOH for production efficiency for adipic acid formed expressedin g/l.h.

                  TABLE 1                                                         ______________________________________                                                              DC %                                                           Reac-          of the                                                                              RT %  Peff  RT %                                         tion    O2 in  cyclo-                                                                              towards                                                                             for   towards                                                                             Lty                             Examples                                                                             time    1/min  hexane                                                                              AdOH  AdOH  olone %                               ______________________________________                                        Example 1                                                                            125 min 0.45   23.2   68.5  54.7 15.2  87.7                            Example 2                                                                            130 min 0.45   23.9   67.9  53.8 14.9  86.9                            Example 3                                                                            133 min 0.44   23.5   68.3  52.0 14.5  87.1                            Example 4                                                                            130 min 0.44   22.9   67.9  51.5 14.8  86.8                            Example 5                                                                            130 min 0.44   34.6   68.3  55.7 13.9  86.4                            ______________________________________                                    

It is observed that the catalytic activity, the selectivities towardsadipic acid and towards olone (mixture of adipogenic compounds) and thelinearity are conserved over 5 successive oxidations.

COMPARATIVE TESTS a TO e

The first oxidation test is performed with the same amounts of reagentsand catalyst and the same operating conditions as Example 1, but thetreatment of the final reaction mixture is different.

As in Example 1, the acetic phase contains almost all of the acids andlactones formed and the very great majority of the cyclohexanol,cyclohexanone and cyclohexyl acetate which are formed, as well as thewater and virtually all of the catalyst (about 5% of the catalyst, whichwere entrained by precipitation of the adipic acid, are recovered in theacetic acid used for the recrystallization of the adipic acid and thinacetic solution is added to the acetic phase.

The acetic phase is concentrated by heating to 50° C., at a reducedpressure (4 kPa) in order to remove the water and some of the aceticacid, cyclohexane, cyclohezanol and cyclohexanone, until a solutioncontaining 110 g of acetic acid and 35 g of products (essentially thediacids and the cobalt catalyst) is obtained.

This concentrated acetic solution is sent to the reactor with the extracobalt, cyclohexane and acetic acid necessary to reestablish theproportions indicated above for the first test a.

The catalyst is successively recycled 4 times (comparative tests b, c, dand e).

The results obtained with the tests a to e are collated in Table 2below.

                  TABLE 2                                                         ______________________________________                                                              DC %                                                    Compara-                                                                             Reac-          of the                                                                              RT %  Peff  RT %                                  tive   tion    O2 in  cyclo-                                                                              towards                                                                             for   towards                                                                             Lty                             test   time    1/min  hexane                                                                              AdOH  AdOH  olone %                               ______________________________________                                        Test a 122 min 0.46   21.7  65.4  50.1  15.0  86.8                            Test b 125 min 0.44   21.3  69.0  50.6  9.1   84.7                            Test c 130 min 0.36   13.8  71.0  32.5  7.0   84.3                            Test d 181 min 0.30   16.2  71.1  27.4  7.6   87.7                            Test e 223 min 0.21   15.2  69.7  20.5  9.1   85.3                            ______________________________________                                    

It is observed that the overall selectivity is towards adipic acid andolone and the linearity are conserved over 5 successive oxidations. Onthe other hand, if the activity is conserved during the first recyclingof the catalyst, a progressive deactivation of the said catalyst isobserved.

In addition, the crude adipic acid which precipitates during cooling ofthe final reaction mixture is increasingly soiled by the other oxidationproducts whose concentration rises as the recyclings continue, and thisprecipitated adipic acid entrains increasingly more cobalt.

EXAMPLE 6

A solution, which in homogeneous at 100° C., containing 120 g of aceticacid, 3.7 g of succinic acid (31.6 mmol), 7.3 g of glutaric acid (55.3mmol). 29.2 g of adipic acid (200 mmol) and 4 g of cobalt acetatetetrahydrate (16 mmol) is gradually cooled to room temperature.

A solid containing 21 g of adipic acid is separated out by filtrationand a filtrate consisting of 116 g of acetic acids 3.0 g of succinicacid (25.4 mmol), 7.0 g of giutaric acid (53 mmol), 8.2 g of adipic acid(56 mmol) and 4 g of cobalt acetate tetrahydrate (16 mmol) is recovered.

This filtrate in concentrated to dryness and then treated with twice 50ml of hot (56° C.) acetone.

The distribution of the compounds between the acetone extract and theremaining solid residue is indicated in Table 3 below.

                  TABLE 3                                                         ______________________________________                                                                        Solid residue                                 Compounds                                                                             Dry filtrate                                                                            Acetone extract                                                                             after extraction                              ______________________________________                                        Succinic acid                                                                         25.4 mmol 18 mmol (70.9%)                                                                             7.1 mmol (28%)                                Glutaric acid                                                                         53 mmol   36.4 mmol (68.7%)                                                                           16.1 mmol (30.4%)                             Adipic acid                                                                           56 mmol   45.1 mmol (80.5%)                                                                           10.7 mmol (19.1%)                             Cobalt  16 mmol   <0.016 mmol (<0.1%)                                                                         15.9 mmol (99.4%)                             ______________________________________                                    

EXAMPLE 7

A solution of the same composition as that of Example 6 is treated inthe same way in order to separate out, by crystallization andfiltration, the majority of the adipic acid.

The filtrate obtained consists of 115 g of acetic acid, 3.7 g ofsuccinic acid (31 mmol), 7.3 g of glutaric acid (55 mmol), 8.8 g ofadipic acid (60 mmol) and 4 g of cobalt acetate tetrahydrate (16 mmol).

This filtrate is concentrated to dryness and then treated with 5 times100 ml of a hot (70° C.) cyclohexanone/acetic acid mixture (82/18weight/weight).

The distribution of the compounds between the extract of the solventmixture and the remaining solid residue is indicated in Table 4 below.

                  TABLE 4                                                         ______________________________________                                                                        Solid residue                                 Compounds                                                                             Dry filtrate                                                                            Solvent extract                                                                             after extraction                              ______________________________________                                        Succinic acid                                                                         31 mmol   16.4 mmol (52.9%)                                                                           14.5 mmol (46.7%)                             Glutaric acid                                                                         55 mmol   44.8 mmol (81.5%)                                                                           10.1 mmol (18.4%)                             Adipic acid                                                                           60 mmol   42.9 mmol (71.5%)                                                                           17.1 mmol (28.5%)                             Cobalt  16 mmol    0.9 mmol (5.6%)                                                                            15 mmol (93.8%)                               ______________________________________                                    

EXAMPLE 8

A solution, which is homogeneous at 100° C., containing 120 g of aceticacid, 3.4 g of succinic acid, 6.3 g of glutaric acid, 29.9 g of adipicacid and 4.1 g of cobalt acetate tetrahydrate is gradually cooled toroom temperature.

A solid containing 21.2 g of adipic acid is separated out by filtrationand a filtrate consisting of 116 g of acetic acid, 3.2 g of succinicacid (27.1 mmol), 6.2 g of glutaric acid (47 mmol), 8.7 g of adipic acid(60 mmol) and 4.1 g of cobalt acetate tetrahydrate (16.4 mmol) isrecovered.

This filtrate is partially concentrated, in order to give a hot solutionof about 52 g. This hot concentrated solution is treated by running168.5 g of acetone therein. The temperature of the mixture stabilizes to60° C. The mixture consists of a solid and a supernatant liquid, whichare separated while hot.

The distribution of the compounds between the supernatant liquid and theseparated solid residue is indicated in table 5 below.

                  TABLE 5                                                         ______________________________________                                                 Initial  Supernatant   Separated solid                               Compounds                                                                              filtrate liquid        residue                                       ______________________________________                                        Succinic acid                                                                          27.1 mmol                                                                              17.2 mmol (63.5%)                                                                            9.5 mmol (35%)                               Glutaric acid                                                                          47 mmol  42.5 mmol (90.4%)                                                                            4.5 mmol (9.6%)                              Adipic acid                                                                            60 mmol  55.5 mmol (92.5%)                                                                            4.4 mmol (7.3%)                              Cobalt   16 mmol   0.25 mmol (1.5%)                                                                           15 mmol (98.2%)                               ______________________________________                                    

We claim:
 1. Process for recycling a catalyst containing cobalt, in areaction for the direct oxidation of cyclohexane to adipic acid, saidprocess including a step for the treatment of the reaction mixtureobtained during the oxidation of the cyclohexane to adipic acid,consisting of an extraction of at least some of the glutaric acid andsuccinic acid which are formed in the reaction.
 2. Process according toclaim 1 for recycling a catalyst containing cobalt, in a reaction forthe direct oxidation of cyclohexane to adipic acid, in a solventcomprising at least one aliphatic carboxylic acid having only primary orsecondary hydrogen atoms, by a gas containing oxygen, wherein:the crudereaction mixture derived from a prior operation for the oxidation of thecyclohexane to adipic acid is treated in order to remove the morevolatile compounds, optionally after the crystallization and separationof at least some of the adipic acid which it contains, the residueobtained is extracted using a solvent, chosen from ketones, alcohols,esters, various mixtures thereof or mixtures of hydrocarbon andcarboxylic acid, this solvent being capable of dissolving all or a largepart of the diacids contained in the said residue, the extractionresidue thus obtained, containing the larger part of the cobaltcatalyst, is used in a new operation for the oxidation of cyclohexane toadipic acid, after addition of the necessary complements of cyclohexane,of carboxylic acid and, where appropriate, of cobalt catalyst. 3.Process according to claim 2, wherein the crude reaction mixture istreated by distillation, at atmospheric pressure or at reduced pressure,of the more volatile compounds, the carboxylic acid serving as solventin the oxidation reaction, the water formed and certain intermediatecompounds.
 4. Process according to claim 2, wherein the solvent whichserves for extraction of the catalyst from the residue obtained ischosen from acetone, methyl ethyl ketone, cyclohexanone, 1-propanol,2-propanol, 1-butanol, 2-butanol, tert-butanol, cyclohexanol, estersderived from the alcohols mentioned above with aliphatic carboxylicacids having only primary or secondary hydrogen atoms.
 5. Processaccording to claim 1 for recycling a catalyst containing cobalt, in areaction for the direct oxidation of cyclohexane to adipic acid, by agas containing oxygen,the crude reaction mixture, obtained from a prioroperation of oxidation of the cyclohexane to adipic acid, from whichmixture at least some of the intermediate oxidation products, issubjected to at least one extraction using at least one cosolvent orusing a mixture comprising a cosolvent and a carboxylic acid, in thatthere are separated, on the one hand, a mixture containing at least someof the cobalt catalyst, some of the carboxylic acid and possiblyresidual amounts of other compounds and, on the other hand, a solutioncontaining the cosolvent and at least some of the glutaric acid andsuccinic acid which are formed in the oxidation reaction, as well ansome carboxylic acid, and in that the mixture containing at least someof the cobalt catalyst is used in a new operation for the oxidation ofcyclohexane to adipic acid, optionally after the addition of extracobalt catalyst.
 6. Process according to claim 5, wherein the cosolventused is chosen from aliphatic and cycloaliphatic hydrocarbons, ketonesand alcohols.
 7. Process according to claim 2, wherein the aliphaticcarboxylic acid used is acetic acid.
 8. Process according to claim 5,wherein the crude reaction mixture is subjected to cooling to atemperature of 16° C. to 30° C. to bring about the crystallization of atleast some of the adipic acid formed, thus giving either a three-phasemedium comprising a solid phase essentially consisting of adipic acid,an upper cyclohexane liquid phase and a lower acetic liquid phase, orgiving a two-phase medium comprising a solid phase essentiallyconsisting of adipic acid and an acetic phase, followed, if need be,after filtration or centrifugation of the solid, by separation of thetwo liquid phases after settling.
 9. Process according to claim 8,wherein the reaction mixture is concentrated prior to the adipic acidcrystallization operation.
 10. Process according to claim 5, wherein thecrude reaction mixture is removed while hot, the said reaction mixturethen being a two-phase or monophase mixture, two liquid phases: an uppercyclohexane phase and a lower acetic liquid phase are separated aftersettling, if necessary, and the said lower acetic phase or the singleacetic phase is cooled to a temperature of 16° C. to 30° C. to bringabout the crystallization of at least some of the adipic acid formed,which is then separated out by filtration or centrifugation of theacetic phase.
 11. Process according to claim 8, wherein the adipic acidis purified by recrystallization from a suitable solvent, which is thenoptionally added to the acetic phase obtained above.
 12. Processaccording to claim 2, wherein a water/cyclohexane azeotropic mixture isdistilled off during the oxidation reaction, and the cyclohexane is thenreintroduced into the reactor after separation of this mixture bysettling and the crude reaction mixture is treated.
 13. Processaccording to claim 7, wherein the acetic phase, obtained in thetreatment of the crude reaction mixture, is subjected to an extractionwith cyclohexane, obtained after separation by settling, or after havingbeen concentrated by heating to a temperature of 30° C. to 80° C. atreduced pressure.
 14. Process according to claim 13, characterized inthat concentration of the acetic phase reduces this phase to a volumerepresenting from 80% to 10% of its initial volume.
 15. Processaccording to claim 13, characterized in that the acetic phase isconcentrated to dryness, that is to say that all of the acetic acidwhich it contains is removed.
 16. Process according to claim 13, whereinthe compounds separated from the acetic phase during concentrationthereof are recycled in the cyclohexane oxidation step, either in totalor after separation of at least some of the water which they contain.17. Process according to claim 16, wherein the acetic phase is extractedeither with cyclohexane alone or with cyclohexane/acetic acid mixtures,so that the acetic phase subjected to the extraction/cyclohexane mixturehas an overall cyclohexane/acetic acid weight ratio between 1/1 and50/1.
 18. Process according to claim 17, wherein the extraction isperformed one or more times or, within the context of a continuousprocess, by the usual industrial techniques and in that it is carriedout at a temperature ranging up to the boiling point of the solvent orsolvents used.
 19. Process according to claim 17, wherein the extractionoperation gives, on the one hand, a solution which contains at leastsome of the glutaric acid and succinic acid, as well as residual amountsof other by-products, and, on the other hand, a mixture essentiallycontaining the cobalt catalyst, it being possible for this mixture to beseparated by settling.
 20. Process according to claim 19, wherein theseparated cobalt catalyst is recycled in a new cyclohexane oxidationreaction, optionally after a further addition of the cyclohexane tocompensate for the losses suffered during the various treatments of thereaction mixture obtained from the cyclohexane oxidation.
 21. Continuousprocess for the oxidation of cyclohexane to adipic acid, using a gascontaining oxygen, in a liquid medium comprising a carboxylic acid assolvent and in the presence of a catalyst containing at least cobalt,comprising the following steps:a) the actual oxidation of thecyclohexane to adipic acid, b) the reaction mixture derived from theoxidation of the cyclohexane to adipic acid is treated in order toremove the more volatile compounds, optionally after crystallization andseparation of at least some of the adipic acid which it contains, c) theresidue obtained is extracted using a solvent chosen from ketones,alcohols, esters, various mixtures thereof or mixtures of hydrocarbonand carboxylic acid, which solvent is capable of dissolving all or alarge part of the diacids contained in the said residue, d) theextraction residue thus obtained, containing the larger part of thecobalt catalyst, is used in a new cyclohexane oxidation operation a).22. Continuous process for the oxidation of cyclohexane to adipic acid,using a gas containing oxygen, in a liquid medium comprising acetic acidas solvent and in the presence of a catalyst containing at least cobalt,comprising the following steps according to one of claim 5,a) the actualoxidation of the cyclohexane to adipic acid, b) either: b1) cooling ofthe crude final reaction mixture, optionally after concentrating, inorder to crystallize at least some of the adipic acid, separation of thesaid adipic acid by filtration or centrifugation and then, if necessary,separation by settling of the filtrate or of the centrifugate obtainedinto a cyclohexane phase and an acetic phase,or: b2) removal of thecrude final reaction mixture while hot, the said reaction mixture beinga two-phase or a monaphase mixture, its possible separation by settlinginto a cyclohexane phase and an acetic phase, cooling of the aceticphase which has been separated by settling or, where appropriate, of thesingle acetic phase so as to crystallize at least some of the adipicacid, and separation by filtration or centrifugation of the said adipicacid and of the acetic phase, or: b3) removal of the water from thereaction mixture during the oxidation step a), by distillation of thecyclohexane/water azeotrope and optional reintroduction of the distilledcyclohexane into the reactor, followed by treatment of the crude finalreaction mixture according to the variant b1), the said treatment givinga single acetic phase c) concentration of the acetic phase in order toremove therefrom at least the majority of the water, if this has notbeen done in the variant b3), d) extraction of the acetic phase usingcyclohexane or a cyclohexane/acetic acid mixture in an amount such that,in the cyclohexane/acetic phase mixture, the overall cyclohexane/aceticacid weight ratio is between 1/1 and 50/1, e) separation of acyclohexane solution containing at least some of the glutaric acid andsuccinic acid, on the one hand, and of a mixture essentially containingthe cobalt catalyst, on the other hand, f) recycling of the mixturecontaining the cobalt catalyst in a now oxidation reaction a). 23.Process according to claim 3, wherein the more volatile compounds areunreacted cyclohexane.
 24. Process according to claim 3, wherein theintermediate compounds are cyclohexanol and cyclohexanone.
 25. Processaccording to claim 4, wherein the aliphatic carboxylic acids having onlyprimary or secondary hydrogen atoms are those which are used in thecyclohexane oxidation reaction, mixtures of several of the extractionsolvents and mixtures of aliphatic or cycloaliphatic hydrocarbons andcarboxylic acids.
 26. Process according to claim 5, wherein theintermediate oxidation products comprise cyclohexanol and cyclohexanone,some carboxylic acid solvent and water has been separated and from whichmixture at least some of the adipic acid formed has been recovered bycrystallization.
 27. Process according to claim 6, wherein the cosolventis cyclohexane.
 28. Process according to claim 11, wherein the solventis acetic acid or water.
 29. Process according to claim 17, wherein theoverall cyclohexane/acetic acid weight ratio is between 2/1 and 15/1.30. Process according to claim 18, wherein the temperature is between10° C. and 80° C.